The reflectance spectra of orthorhombic GeSe crystals are studied in the region 0.5 to 12 eV in polarized light with E 11 a and E (1 b a t 290 K and a Kramers-Kronig analysis is performed. The thermoreflectance spectra are investigated in the region 1 to 5 eV for E 11 a and E 11 b a t 300 and 90K. The energy band structure calculations of GeSe are carried out by the pseudopotential method, and the spectra of t.he imaginary part of the dielectric constant are calculated by the k * p method. A classification and interpretation of main structures of optical spectra of GeSe by direct interband transitions a t critical points I', U, T and high symmetry lines B' V, A is presented on the basis of calculated band structure and selection rules for optical dipole transitions of orthorhombic A4B6 crystals. I/ZCcJIenOBaHbI CneKTpbI OTPaHEeHHfl OpTOpOM6H~IeCKOrO HpHCTanna GeSe B CneKTpNIb€IO# o 6 n a c~~ OT 0,5 A 0 12 ev B nOJIflpH30BaHHOM CBeTC C E I] a A E I] b npPi TeMnepaTypC! 290 K A nposeneH ~H~J I A~ HpaMepca-Kpomira. kI3yqe~b1 cnewpbr TepMooTpameHufl B AHTepsane OT 1 no 5 eV nns non~pn3a1@ E 11 a A E 11 b n p~ 300 EI 90 K. nposeHeHbI paClieTb1 30HHOfi CTPYKTYPbI &Se MeTOnOM IICeBAOnOTeHqHana H CneKTpOB MHHMOfi gaCTH AH3JIeKTPAYeCKOfi IIPOHH4aeMOCTki k * P-MeTOllOM. Ha OCHOBe PaCCWITaHHOfi ~O H H O~~ CTPYKTYPH H npamn o~6opa n n~ nnnonbHbIx OnTmecwix nepexonoB B OPTO-P O M~M V~C K H X HpHcTannax A4B6 nposeneHa xnaccH@Kaqax H AHTepnpeTaqufl OCHOBHHS CTPYKTJ'P OIITHqeCKAX CneKTpOB GeSe IlpIIMbIMH MeHU(y3OHHbIMH nepeXO&3MH B KPATII-qeCKHX TOqKaX r, u H T, a TaKHEe Ha JIHHHIIX BbICOKOfi CAMMeTpAA B', v H A.
Raman scattering in ε-GaSe has been investigated at room temperature in a diamond anvil cell by using helium gas as a pressure transmitting medium. The first observation of the solid − solid phase transition under pressure from a hexagonal structure at ambient pressure to a NaCl-type at around 29.2 GPa is reported. A photoinduced change in the pressurised crystal was observed. The transition under pressure was found to be irreversible at room temperature. It was shown that the pressurised form of ε-GaSe at ambient conditions most probably belongs to the γ-type as characterised by an increased intensity of the Raman modes at 234 cm −1 and 250 cmGallium selenide is a layer semiconductor. Four distinct modifications, the β-, the ε-, the γ-and the δ-types were reported for GaSe, differing by the stacking order of the two-dimensional Se−Ga−Ga−Se sheet [1]. The differences between these types arise from the stacking of layers as well as from the number of layers per unit cell. The ε-modification (D 3h 1 space group (SG), containing two layers per unit cell) is the most common and best studied. Up to now the existence of the β-modification (D 6h 4 SG, two layers per unit cell) has not been confirmed. The γ-type has three layers (C 3v 5 SG), and the δ-type contains four layers per unit cell (C 6v 4 SG). Normally the crystals grown by the Bridgman method are a mixture of ε-and γ-types, with a predominant content of the former.Most common and well investigated among the different polytypes is ε-GaSe, which is one of the promising materials for far-infrared conversion applications [2]. The properties of ε-GaSe at ambient conditions (room temperature, atmospheric pressure) are reasonably well known. Several Raman scattering experiments under pressure have already been performed on GaSe [3−6]. The pressure dependence and the mode Grüneisen parameters for Raman-active phonons of ε-GaSe have been measured up to 1 GPa [3, 4], 3.5 GPa [5], and 8 GPa [6]. Recently [7,8] high-pressure X-ray powder diffraction studies have been made on ε-GaSe. It was shown that at pressures of 29 GPa [7] and of around 25 GPa [8] the crystal undergoes a phase transition (PT) to the NaCl-type structure. The reason for this discrepancy is
The dielectric susceptibility of layered TIInS2 was studied in the temperature range of successive phase transitions. Thermal hysteresis was observed in the incommensurate phase. It was shown that after annealing the crystal at a fixed temperature within the incommensurate phase, the existing temperature interval of this phase reveals noticeable broadening. The thermal memory effect is discussed using a defect density wave model.
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